1. Field of the Invention
The present invention relates to a system for conducting in situ and non-destructive measuring of the thickness of coatings, and more particularly to a miniaturized pipe inspection system for measuring corrosion and scale in small pipes that uses backscattering of gamma rays to inspect corrosion and scale substrates disposed inside small pipes, which may be twisted, bent, or have irregular cross-sections.
2. Description of the Related Art
To detect corrosion in small pipes, such as heat exchanger pipes, radiographic techniques are sometimes used in which one radioactive source is inserted inside one tube and radiographic film is inserted inside an adjacent or nearby tube. This technique is time-consuming and does not provide adequate information, and has a limitation on the size of radiographic films that can be used. Moreover, a high radioactivity source, on the order of many Curies, is usually used, which is associated with radiation hazard in the inspection process.
Gamma ray attenuation is also used in which a radioactive source is inserted in one pipe and a radiation detector is put into an adjacent or nearby tube and attenuation in the tube wall is measured. The detector used in these cases is usually a Geiger-Muller counter. Besides being time-consuming, this technique cannot predict which pipe has the defect, the one in which the source is inserted or the one in which the detector is inserted. And because Geiger counters are gas-filled detectors, it usually has very low efficiency, typically several orders of magnitude compared to solid scintillation detectors. Moreover, Geiger detectors are counters only, providing no information on the spectrum of attenuated rays, and indiscriminately responding to both direct and scattered radiation. This tends to produce errors in the registered counts. Radiation detectors/spectrometers, such as scintillation detectors, are more useful, but cannot be inserted inside small diameter tubes because the photo-multiplier tube (PMT) and associated electronics are often much larger in size than the tube diameter. Solid state detectors are very expensive, and their associated electronic components are large, expensive, and not useful for field work. Other techniques are used, in which a neutron source is inserted inside the pipe to activate the pipe wall or surrounding materials, and then measuring the induced radioactivity in walls by gamma detector. Again, this technique can be used in large sewage buried pipes, but cannot be used in pipes of heat exchangers. Copper, nickel or iron, the material of the tubes, cannot be activated easily by a small neutron source because of their very small neutron absorption cross section. Moreover, gamma rays need to be measured by high efficiency detectors that cannot be inserted inside heat exchangers.
Eddy current and ultrasound techniques can be used, but the inside of the pipe needs to be well prepared and clean, which makes the inspection process more difficult and time-consuming.
It should be clearly mentioned here that none of the existing techniques work for twisted pipes in heat exchangers. Also, it is very difficult to use existing techniques for inspecting pipes whose cross section is not circular, or for inspecting bent pipes.
Thus, a miniaturized pipe inspection system for measuring corrosion and scale in small pipes solving the aforementioned problems is desired.